The present invention relates to an earth boring apparatus, and to a drill rod for use in such an earth boring apparatus.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
The introduction of boreholes in the ground is implemented by boring apparatuses having a drill head arranged on a drill rod on the front side and propelled into the ground by means of the drill rod. The pressure forces required for the propulsion are hereby applied by a driving device which is arranged above ground or an excavation pit near the ground surface. In general, the drill head—in addition to the pressure forces—is simultaneously caused to rotate, with the rotation being produced either by a down-the-hole motor, a motor which is arranged in the area of the drill head and transmits the rotation movement only to the front of the drill head, or also by the driving device arranged on the surface or in the excavation pit, with the rotation movement again being transmitted by the drill rod.
In particular, when producing horizontal boreholes, which normally are realized starting from a defined surface-near starting point to a surface-near destination point which is also defined beforehand, it is required to permanently monitor the course of the borehole in order to undertake a correction, if need be, when encountering an excessive deviation from the previously determined desired course. Controllable horizontal boring apparatuses have been developed for this purpose.
The supervision of the drilling course requires a localization of the position of the drill head within the earth. A radio transmitter has oftentimes been used which is arranged within the drill head and sends out radio signals which are received by a receiver unit above ground and analyzed for determining the position of the drill head. The radio transmitter has to receive electric energy for this purpose. This may be realized using batteries which are also arranged in the area of the drill head. The use of batteries as energy sources for the radio transmitters involves however much maintenance because they need to be replaced or charged regularly. This need for maintenance should preferably be avoided. Furthermore, depending on the drilling depth, the earth being bored through, and the type of utilized receiver, fairly strong radio signals must be transmitted which require the availability of a respectively powerful energy source. To configure such an energy source in the form of batteries is accompanied with disproportionate economic effort. Because of these disadvantages relating to the use of batteries as an energy source for the radio transmitter, arranged in the drill head, the provision of an external energy source, i.e. above ground, is proposed. Oftentimes, this involves a 12 volt or 24 volt battery by which primarily the boring drive and/or a control device of the earth boring apparatus is supplied. The electrical energy must then be transmitted between the external energy source and the radio transmitter by normally using an electrical conductor (cable or cable string) installed within the drill rod.
Such an electrical conductor can simultaneously also be used for transmitting signals from sensors which are arranged in the area of the drill head, to an evaluation unit arranged on the ground surface. For example, in the area of the drill head of a horizontal boring device, rolling sensors, inclination sensors as well as sensors for detecting electromagnetic fields are used as they are emitted by current conductors installed in the earth.
Since the boring lengths of horizontal boreholes can frequently be several hundred meters and the vertical boreholes may sometimes also have lengths of several thousand meters, inevitably drill rods must be used which are composed of individual interconnected rod sections. In accordance with the advance of the borehole, the individual rod sections are successively attached to the already existing boring string.
In dependence on the earth layers to be bored through, it is further normally necessary to assist the boring propulsion by applying a drill fluid. The drill fluid primarily has the purpose of lubricating the drill head, to soften the earth in front of the drill head, and thereby to improve the boring propulsion and to flush out the borings removed by the drill head through the annular space formed by the drill rod and the borehole wall. The drill fluid, which as a rule is fed to the drill head through the interior space of the hollow drill rod, is discharged through outlet openings in the area of the drill head. As drill fluid an aqueous solution of bentonite and possibly additional additives has been found useful in practice. Such a bentonite drill fluid has a pH-value of less than 7, i.e., it behaves chemically like an acid. Moreover, drill fluids and especially bentonite drill fluids are more or less electrically conductive, wherein the electrical conductivity and the charge transport are effected by the directional movement of ions. Consequently, the drill fluids are electrolytic.
The placement and connection of a new rod section to the existing drill string is usually carried out by machines to keep the time expended within limits. However, this is problematic when an electric conductor extends within the drill string for supplying a radio transmitter, arranged in the drill head, with electrical energy or for transmitting the signals from the sensors, arranged in the area of the drill head, to an evaluation unit arranged above ground. In order to make it unnecessary to thread the individual rod sections onto a cable of a length corresponding to the intended drill course, it is normally provided to implement the electrical conductor in the manner of a string, i.e., to progressively extend the conductor through attachment of a new segment. This has the result that each time a new rod section is to be attached and connected to the drill string, initially an appropriate segment of the electrical conductor must be connected to the then free end of the conductor string arranged within the already bored drill string. This is very time-consuming because normally there is not only the need to establish the contact between the last segment of the conductor string and the new conductor segment, but the connection point must also be sealed in a watertight manner (insulated) in order to prevent the electrically conductive drilling mud from causing an electric shorting between the conductor and the rod casing of the drill rod serving as return conductor. In view of the ohmic resistance of the drill fluid which is very high in relation to the electrical conductor and the rod casing of the drill rod, such an electrical shorting leads in the case of an incorrect sealing normally only to more or less great electrical losses, but not to a failure of the energy supply or signal transmission. However, in the event the contact points are leaky there may be the problem in the long run that an electrolysis takes place as a result of the short circuit which is accompanied by corresponding electrolytic reactions at the electrodes, i.e., the non-insulated contact points of the electrical conductor as well as the rod casing of the drill rod. Since the electrical conductor or the non-insulated contact point constitutes an anode charged with a positive voltage potential, an anodic reaction takes place at this anode which is characterized by the deposition of an oxide layer. In contrast thereto, at the cathode, i.e., the rod casing of the drill rod, a cathodic reaction appears as a gas formation.
The anodic reaction at the non-insulated contact points of the electrical conductor poses in the medium and long run a significant problem because the oxidation can deteriorate and possibly completely interrupt the electrical contact between the segments of the conductor so that the energy supply for the radio transmitter or the signal transmission of the sensors of the drill head is no longer operational. Since both functions are necessary for carrying out the boring project, it is necessary in such a case to pull back the drill string step by step and to examine the contact points of the electrical conductor in order to find the faulty location. This is very time-consuming.
It would therefore be desirable and advantageous to provide an improved ground boring apparatus to obviate prior art shortcomings and to prevent the danger of a faulty operation as a result of oxidized contact points of the electrical conductor.